BRUSSELS, March 10 (Reuters) - The European Union and Japan are seeking to be excluded from U.S. steel and aluminium tariffs in talks with U.S. President Donald Trump's trade envoy on Saturday, with the EU warning it will retaliate if Washington does not relent.

Trump set import tariffs on Thursday of 25 percent on steel and 10 percent on aluminium. But he exempted Canada and Mexico and held out the possibility of excluding other allies .

The EU executive said it remained unclear how an exclusion would work. It said it wanted details from Saturday's talks with U.S. Trade Representative Robert Lighthizer in Brussels but that it did not expect the meeting to resolve all the problems.

"We need a dialogue with the United States. It's clear. We need clarity," European Commission Vice President Jyrki Katainen said on Friday.

"We are an ally, not a threat," he said, rejecting any hint that the bloc's exports threatened U.S. national security - Trump's justification for imposing the tariffs.

Lighthizer's meetings with EU trade chief Cecilia Malmstrom and separately with Japanese Trade Minister Hiroshige Seko were long planned, but they took on added urgency as allies warned the U.S. moves could provoke a global trade war.

Japan, the United States' top economic and military ally in Asia, is among the other nations seeking to join the exemption list.

In talks, the United States should not expect any EU concessions to win an exemption, EU officials said.

"This is not a trade negotiation," Katainen said. "Now we are talking about unilateral action against international rules, and we want to sort it out before it really becomes a problem."

The European Commission, which coordinates trade policy for the 28-nation EU, the world's biggest trading bloc, has said it is ready to impose safeguards, tariffs or quotas to protect its own steel and aluminium industries from products diverted to Europe because of the U.S. measures.

It has already started monitoring incoming metal flows to see whether a surge occurs.

The EU is also maintaining a threat of counter-measures that would target U.S. imports ranging from maize to motorcycles, and may publish its list next week to allow industry and other interested parties to give their input. Under World Trade Organization rules, such counter-measures have to be in place within 90 days of the U.S. tariffs entering force.

European steel and aluminium associations have warned that the U.S. tariffs could cost their sectors thousands of jobs. (Reporting by Phil Blenkinsop, Robin Emmott and Alissa de Carbonnel, writing by Alissa de Carbonnel, editing by Larry King)